When you select one case to route, the user view by default is the "Private Queue User Lookup View" this it is fine.
But if you select more than one, the view by default is "User Lookup View" this allow to the user route or assign the case to an user out this queue that can be private or not. We think that two cases in a private queue should be only routing to users that belong to this queues and not any user.
Maybe is more interesting if the user will could use a custom view.